1. Field of the Invention
This invention relates to a balance mechanism for a press machine.
2. Discussion of Prior Art
To improve productivity and return on capital investment, the operating speed of press machinery, as well as other machinery, has been steadily increased over the years. As the speed increases, the greater acceleration of the moving parts gives rise to progressively greater shaking forces. These shaking forces are proportional to the square of the operating speed. The mass of moving parts can be reduced to some extent in order to reduce shaking forces, but reduced strength and stiffness can impair the quality of the components produced. Also, at very high speeds, extra strength and stiffness are required to withstand the extra load imposed by the high shaking forces.
The shaking forces may reduce accuracy of tool alignment and give rise to tool wear, and possibly tool damage by fatigue, leading to a reduction in product quality. Vibration may cause machine parts to resonate. Furthermore, vibrations transmitted to the ground can damage foundations and cause disturbance to nearby machines.
If further increases in speed are to be achieved, it is very important to reduce shaking forces. Shaking forces may be reduced by dynamic balancing and some prior art approaches to such balancing will now be described.
In general, in a machine having an oscillating member, there will be shaking forces and shaking moments. During each cycle of the machine, the shaking forces will vary in both magnitude and direction while the shaking moments will vary in magnitude. The shaking forces can be resolved into a horizontal force and a vertical shaking force.
The horizontal shaking force can be expressed as a Fourier series of sinusoidal shaking forces. In this series, there will be a fundamental component having a frequency which is equal to the frequency of oscillation of the oscillating member. There will also be harmonic components at frequencies which are equal to whole number multiples of the frequency of oscillation. The vertical shaking force may be expressed as a Fourier series of sinusoidal shaking forces in a similar manner.
The fundamental components of the Fourier series are known as the primary shaking forces. These primary shaking forces are usually the significant components of the shaking forces. The first harmonic components are known as the secondary shaking forces. Although usually much smaller in magnitude than the primary shaking forces, the secondary shaking forces can also be significant. Higher harmonic components are not usually significant. However, some machines generate secondary (or higher) shaking forces which are greater than the primary shaking forces.
In designing a balance mechanism, it is often essential to balance the primary shaking forces. Sometimes, it is acceptable if the secondary shaking forces or the shaking moments remain unbalanced. However, for some machines, it may be useful to balance secondary (or higher) shaking forces without balancing the primary shaking forces.
The analysis of shaking forces in a press machine for making can bodies is discussed in a paper entitled "The Computer-aided design of a retrofit package to reduce the shaking forces in a metal can bodymaker machine", by M R Askari and T H Davies. This paper was read at a conference entitled "High Speed Machinery", held at 1 Birdcage Walk, Westminster, London on Sep. 24, 1988 and organised by the Institution of Mechanical Engineers.
Referring now to FIG. 1, there is shown a simple machine having a reciprocating member 10 mounted for sliding movement, a crankshaft 11 and a connecting rod 12. For simplicity, the connecting rod 12 is assumed to be lightweight. For a real machine, the weight of a connecting rod can be significant. The reciprocating movement of the member 10 gives rise to a horizontal shaking force. The movement of the reciprocating member 10 departs to some extent from simple harmonic motion and this departure may be understood from the following discussion. When the member 10 is at the extreme right hand end of its travel, the crankshaft 11 and connecting rod 12 are rotating in opposite directions. In contrast, when the member 10 is at the extreme left hand end of its movement, the crankshaft 11 and connecting rod 12 are rotating in the same direction. Consequently, the magnitude of the acceleration of member 10 will be slightly greater when it is at the extreme right hand end than when it is at the extreme left hand end of its movement. In contrast, if the member 10 were performing pure simple harmonic motion, the magnitude of the acceleration would be equal at both ends of the movement. It is this departure from simple harmonic motion that gives rise to the secondary shaking forces as well as the higher harmonic components.
Referring now to FIG. 2, there is shown a balance mechanism 15 for balancing the member 10. This balance mechanism 15 comprises a member 16 slidingly mounted for reciprocating movement on the opposite side of the crankshaft from the member 10. The member 16 is connected to the crankshaft 11 by a further lightweight connecting rod 17. The connecting rod 17 is mounted on the crankshaft 11 at a position which is axially spaced from the connecting 12. The reciprocating member 16 balances the primary shaking force as well as the secondary shaking force and higher harmonic components which arise due to the movement of the member 10. However, because the connecting rods 12 and 17 are axially spaced from each other, the balance mechanism introduces a shaking moment.
Referring now to FIG. 3, there is shown another balance mechanism 20 for balancing the movement of the member 10. The balance mechanism 20 comprises a balance weight 21 mounted for sliding reciprocating movement on the same side of the crankshaft 11 as the member 10. The member 21 is connected to the crankshaft 11 by a lightweight connecting rod 22, and the connecting rods 12 and 22 are axially spaced from each other. In the balance mechanism 20, the member 21 balances the primary shaking force due to the movement of member 10. However, because both the members 10 and 21 are on the same side of crankshaft 11, they give rise to secondary shaking forces which are in phase with each other and, consequently, the balance mechanism 20 causes the secondary shaking forces to be doubled. Also, because the connecting rods 12 and 22 are axially spaced, the balance mechanism 20 gives rise to a shaking moment. If the shape of the connecting rods 12,22 were altered to bring the members 10,21 into line with each other, this would eliminate the shaking moment but it would introduce bending moments in the connecting rods.
Balance mechanisms of the type shown in FIGS. 2 and 3 suffer from various disadvantages. The masses of the additional moving members increase both the overall inertia of the machine and friction losses. This increases the required capacities of the drive motor, transmission, clutch, brake and flywheel. Of these, the increase in the braking requirement is usually the most important. For example, a high speed press must usually be able to stop in half a machine cycle to prevent tooling damage in the event of a workpiece becoming jammed. Also, when designing a balance mechanism of the type shown in FIGS. 2 or 3, it is often difficult to find a suitable location for the components of the balance mechanism.
In a type of balance mechanism known as a Lanchester balance mechanism, two or more balance weights are mounted eccentrically on shafts which are geared together. Three examples of this type of balance mechanism will now be described with reference to FIGS. 4 to 6.
Referring now to FIG. 4, there is shown a machine having a member 30 mounted for reciprocating movement and driven from a crankshaft 31 via a lightweight connecting rod 32. This machine has a balance mechanism 33 comprising a balance weight 34 mounted eccentrically on the crankshaft 31 and a balance weight 35 mounted on a further shaft 36. The crankshaft 31 and the shaft 36 are connected together by gear wheels 37 and 38 so that they rotate in opposite directions and at the same speed. The balance weights 34 and 35 are equal. The balance weights 34 and 35 balance the primary shaking forces which arise due to the movement of the member 30. However, the secondary forces caused by the member 30 are not balanced and the balance weights 34 and 35 give rise to a shaking moment.
Referring now to FIG. 5, there is shown a machine having a member 40 mounted for reciprocating movement and driven by a crankshaft 41 via a lightweight connecting rod 42. The machine has a balance mechanism 43 which comprises a first balance weight 44 mounted eccentrically on crankshaft 41 and second and third balance weights 45 and 46 mounted eccentrically on a pair of shafts 47 and 48. The crankshaft 41 and shafts 47 and 48 are connected together by a set of gear wheels 49, 50 and 51 so that the shafts 47 and 48 rotate in the opposite direction to crankshaft 41. Each of the balance weights 45 and 46 has a mass equal to half the mass of balance weight 44. The balance mechanism 43 balances the primary shaking forces which arise due to the movement of member 40 without introducing a shaking moment. The secondary shaking forces remain unbalanced.
Referring now to FIG. 6, there is shown a machine having a member 52 mounted for reciprocating movement and driven by crankshaft 53 via a connecting rod 54. The machine has a balance mechanism 55. The balance mechanism 55 comprises a balance weight 56 mounted eccentrically on crankshaft 53, a pair of balance weights 57 and 58 mounted eccentrically on a pair of shafts 59 and 60, and a further pair of balance weights 61 and 62 mounted eccentrically on a pair of shafts 63 and 64. The crankshaft 53 and shafts 59 and 60 are provided with gear wheels 65, 66 and 67. The gear wheels 65, 66, 67 cause the shafts 59 and 60 to rotate in the opposite direction to, but at the same speed as, the crankshaft 53. The shafts 63 and 64 are provided with gear wheels 69 and 70. Gear wheels 69 and 70 together with gear wheel 65 and 66 cause shafts 63 and 64 to rotate in opposite directions to each other and at twice the speed as crankshaft 53. Each of the balance weights 57 and 58 has a mass equal to one half of that of balance weight 56. The masses of balance weights 61 and 62 are equal and much smaller than that of balance weight 56.
The balances weights 56, 57 and 58 together balance the primary shaking forces which arise as a result of movement of the member 52 without introducing a primary shaking moment. The balance weights 61 and 62 balance the secondary shaking forces which arise as a result of movement of member 52. The unsymmetrical arrangement of weights 61, 62 introduces a secondary moment.
Balance mechanisms of the Lanchester type as exemplified in FIGS. 4 to 6 suffer from disadvantages which are similar to those of the balance mechanisms of FIGS. 2 and 3. The balance weights increase the required capacities of the main motor, transmission, clutch and brake of the machine in which they are installed. Also, the ideal positions of the rotating shafts which carry the balance weights may be inconvenient and, sometimes, it may be necessary to provide a chain of gear wheels to drive the shafts from the crankshaft. A further disadvantage is that gear drives are noisy and prone to backlash and wear.